1. Field of the Invention
The invention relates to a self-regulated synchronous rectifier. The invention relates in particular to a self-regulated synchronous rectifier used in an AC/DC or DC/DC converter.
The invention relates to a synchronous rectifier regulated by coupled winding or self-regulated by symmetrical or asymmetrical direct energy transfer. In the remainder of the text, the expression "self-regulated synchronous rectifier" also means "synchronous rectifier regulated by coupled winding".
2. Description of the Prior Art
Asymmetrical conversion systems including an initial voltage source discharging into a transformer primary connected in parallel with a main switch are known in the art. The secondary of the transformer is connected in cascade with a self-regulated synchronous rectifier and a filter. The output of the filter discharges a regulated DC voltage into an application. In a conversion system of the above type the role of the self-regulated synchronous rectifier is:
to deliver to the application, via the filter, the energy transferred by the transformer in the on period of the main switch, and PA1 to block the transfer during the off period of the main switch, the application being powered by the coil of the filter during the off period of the main switch. PA1 a first and a second output of the rectifier, PA1 a first MOSFET connected between the first secondary transformer end and the first output of the rectifier and having its gate connected to the second end of the secondary of the transformer, and PA1 a second MOSFET connected between the first output of the rectifier and the second output of the rectifier and having its gate connected to the first transformer end. PA1 first and second rectifier inputs respectively connected to the first and second transformer ends, PA1 first and second rectifier outputs, the second rectifier output being connected to the second rectifier input, PA1 a direct MOSFET connected between the first rectifier input and the first rectifier output and having a gate connected to the second rectifier input and in series with the gate protection circuit, and PA1 a freewheel MOSFET connected between the first rectifier output and the second rectifier output and having a gate connected to the first rectifier input and in series with the gate protection circuit. PA1 first and second rectifier inputs respectively connected to the transformer ends of the first sub-winding defining a first subsystem, PA1 first and second rectifier inputs respectively connected to the transformer ends of the second sub-winding defining a second subsystem, and PA1 first and second rectifier outputs, the second rectifier output being connected to the second rectifier input of the first subsystem.
An asymmetrical self-regulated synchronous rectifier includes two MOSFETs adapted to perform the above two functions. For example, the asymmetrical self-regulated synchronous rectifier includes:
The secondary voltage of the transformer controls the two MOSFETs.
For economic reasons, manufacturers wish to develop converters accommodating a wide range of input voltage in one and the same product. This implies that the secondary voltage of the transformer also varies within a wide range. However, the secondary voltage of the transformer is also the signal at the gate of the MOSFETs of the rectifier. There are limits on the voltages that can be applied to the gates of MOSFETs. If too high a voltage is applied to the gate of a MOSFET, the MOSFET may be destroyed. Protecting MOSFETs from an overvoltage at the gate by connecting the gate in series with a passive voltage divider bridge is known in the art. However, the voltage divider causes high losses if the transformer secondary voltage is too low or too high. The voltage applied to the gate of the MOSFETs is too low or too high for optimum control of the MOSFETs, i.e. with an optimum control dynamic range which is most economical in terms of losses. As a result, for low output voltages, the feasible power transferred is very low and for higher output voltages the dynamic range of the input voltage is small.
One object of the present invention is to propose a self-regulated synchronous rectifier in which the gate is protected against gate overvoltages, allowing wide variation of the input voltage combined with optimum performance in terms of output current and voltage.